1. Field of the Invention
The invention relates to a filtration media containing nanocrystals made of metal oxide dispersed in a binder matrix for the removal of microorganisms and other contaminants from water.
2. Description of Related Art
Drinking water, in some locations world-wide, contains bacteria and viruses that are harmful to humans, in many cases, rendering the water unfit for consumption. There are a variety of different techniques and equipment that can reduce bacteria and viruses to certain acceptable performance levels, such as ceramic filters, sub-micron filters, ion exchange resins, ultra-violet lights, ozonators, distillation equipment, and other apparatus. Microfiltration generally presents significant drawbacks because of the large pressure drops involved and because of the limited capacity of the microfilters. With bacteria having sizes of around 0.1 micron, such as B. Diminuta, the performance of microfilters is generally very poor, and clogging takes place in a short time. Consumers who use these filters to reduce bacteria generally must rely on increased pressure drop as the only indication that it is time to replace the microfilter. There is no reliable method to determine whether the filter will last 10, 50, 100 or 1000 gallons, or what the remaining capacity of a filter in use might be. Turbidity and the presence of other contaminants than microorganisms can affect the surface of the microfilter, which creates some limitations on the use of the filter. Ultra-violet lights are affected by scale buildup on the lamp sites and frequency changes that may effect their performance in bacteria reduction, and UV wavelength sensors are very expensive.
Filtration media are often assigned a xe2x80x9cratingxe2x80x9d based upon the size of particulates that can be removed from water using these filters. Typical testing to establish these ratings include NSF Class 1 Particulate and NSF 53 AC Dust testing. Reducing the ratings (desirable, because it indicates that smaller particles can be produced) generally requires the use of specialized particles having very small pore sizes. These particles become difficult and expensive to produce, so that decreasing the nominal rating of the filtration media is limited by the expense of the particles necessary to include in the media. In addition, filters that have submicron ratings, and which function by occlusion, have very short lifetimes. For example, a 0.2 micron rated filter of approximately 3 in. diameter and 10 in. length filtering New York City water at 1 gpm will suffer reduced capacity and significantly increased pressure drop after filtering only 100 gallons of water.
Recent advances in xe2x80x9chybridxe2x80x9d materials, i.e., nanostructured materials that contain both organic and inorganic components or moieties, has led to the development of filtration materials capable of achieving submicron level removal of particulates as well as removal of microorganisms, but that are capable of operating at high flow rates and for extended periods of time without substantial degradation of performance. The invention described herein is one such material.
It has been found that combining nanocrystals of metal oxides, such as zinc oxide or titanium oxide or mixtures thereof, encapsulated in or impregnated into a binder matrix. The binder matrix may be a polymeric material, and the metal oxide nanocrystals may be optionally mixed with carbon and/or other organic particulates. The inclusion of the metal oxide nanoparticles significantly decreased the micron rating of the filtration material as compared to the same material without the nanoparticles, and provided a material that is capable of reducing levels of microorganisms, such as bacteria, including those having an average particle size ranging from about 0.1 to about 1 micron, at an efficiency of 99.999%.
Without wishing to be bound by any theory, it is believed that the filtration media functions to remove microorganisms without significant size exclusion of the microorganisms. Regardless of the exact mechanism by which the material functions, it allows the preparation of a filtration media that is capable of removing submicron contaminants at extremely high efficiency. The inclusion of metal oxide nanoparticles in the filtration media allows the use of binder and, e.g., carbon particulates suitable for achieving a micron or larger nominal rating, but in fact achieving submicron performance without diminished lifetime. By contrast with the 0.2 micron filter described above, a similar filter including metal oxide nanoparticles can process over 1000 gallons of the same water at the same flow rate with less than a 30% pressure drop at the end of processing.
According to the invention, nanocrystals of metal oxides having particles sizes ranging from approximately 20 nm to 400 nm are incorporated into a filtration media containing a binder matrix. Desirably, this filtration media also contains some form of additional particulate, such as activated carbon, and the binder is desirably a polymeric material. The resulting filtration media is capable of destroying bacteria and other organisms having sizes below 1 micron.
The filtration media of the invention can be viewed as a microcoating of metal oxide nanocrystals on the surface of a polymeric binder (including internal surfaces, such as those provided by pores within the binder matrix). The metal oxide nanocrystals are included in amounts ranging from approximately 0.1% up to about 10% by weight, based upon the weight of the entire filtration media. Suitable metal oxides include, but are not limited to, zinc oxide, copper oxide, and titanium dioxide. Other metal oxide nanocrystals may also be suitable, and this can be determined by preparing suitable filter blocks containing these metal oxide nanocrystals, as described herein, and testing the blocks against submicron particles and against microorganisms, as described herein, to determine their suitability.
The nanocrystals are believed to interact with the binder, which is typically a polymeric binder, such as high density polyethylene or low density polyethylene or a mixture thereof. The nanocrystals are typically combined with the polymer and, before or after the addition of other optional components such as activated carbon, heated to a temperature ranging from about 150xc2x0 C. to about 250xc2x0 C. The nanocrystals can be incorporated into the polymer by, e.g., high-speed shear mixing for approximately 10-30 minutes in the mixer. The nanocrystals and the polymeric binder in particulate form are simply added to the mixer in the requisite quantities, and mixed. Activated carbon and optionally zirconia can then be added. The order of addition is not critical, however it is generally desirable to add the nanocrystals to the binder prior to adding other components and prior to heating, in order to assure complete mixing. The resulting mixture is then heated to raise the temperature of the polymeric binder. In general, the polymeric binder material containing the metal oxide nanocrystals is heated slowly to form the filtration media. Polymeric binders containing the metal oxide nanocrystals are then heated from about 30 minutes to about 6 hours at an approximate temperature of 550xc2x0 F. in order to form a block of filtration media.